Amplifier circuitry

ABSTRACT

This application relates to amplifier circuitry, in particular class-D amplifiers, operable in open-loop and closed-loop modes. An amplifier ( 300 ) has a forward signal path for receiving an input signal (S IN ) and outputting an output signal (S OUT ) and a feedback path operable to provide a feedback signal (S FB ) from the output. A feedforward path provide a feedforward signal (S FF ) from the input and a combiner ( 105 ) is operable to determine an error signal (ε) based on a difference between the feedback signal and the feedforward signal. The feedforward comprises a compensation module ( 201 ) configured to apply a controlled transfer function to the feedforward signal in the closed-loop mode of operation, such that an overall transfer function for the amplifier is substantially the same in the closed-loop mode of operation and the open-loop mode of operation.

TECHNICAL FIELD

The field of representative embodiments of this disclosure relates tomethods, apparatus and/or implementations concerning or relating toamplifier circuitry, and in particular to class-D amplifiers and methodsof operation thereof.

BACKGROUND

Many electronic devices include amplifier circuitry, for instance foramplifying an input signal to provide a driving signal to drive atransducer, which could be an audio transducer, such as a speaker,whether of the host device or some accessory which is removablyconnected to the host device, or some other transducer, e.g. a linearresonant actuator for generating haptic feedback.

As mixed-signal design moves to deep sub-micron nodes, which areselected to enable digital processing features and minimize area,conventional analogue circuits do not scale well. Analogue amplifiercomponents can thus have a large impact on the area and power of thecircuitry. Digital circuitry scales with process node, making a digitalClass D architecture attractive and allowing the design to benefit fromthe programmability and adaptability of a digital implementation. Aclass-D amplifier with a digital modulator can thus be advantageouslyused in a variety of applications.

A standard digital modulator has a well-defined transfer function, forinstance flat and with a defined gain over some pass-band range offrequency. Ideally this digital-domain signal processing gives highperformance and avoids analogue circuitry with its non-zero signaldegradation associated with noise, component mismatch and non-linearity.However, the performance of a class-D amplifier may be limited byanalogue effects in an output driver stage of the amplifier. Forexample, output driver transistor on-resistance, finite rise and falltimes, propagation delays and output impedance may impact performance.In addition, any power supply ripple will cause a proportional gainvariation of the output driver stage, leading to added noise in theoutput.

Feedback techniques may be used to suppress signal distortion arisingfrom these causes. A class-D amplifier may thus have a feedback path forproviding feedback from the output of the amplifier. The feedback signalmay be subtracted from the input signal to provide an error signal,which may be filtered, and the filtered error signal combined with theinput signal upstream of the digital modulator to compensate for thedownstream distortion. To allow for digital processing of the feedbackand error signals, the feedback path comprises an analogue-to-digitalconverter (ADC). The performance of such an amplifier circuit can belimited by the noise, resolution and linearity of the ADC.

In some implementations, to optimize the performance across the inputsignal range, a digital Class-D amplifier can be designed to operable inboth open-loop and closed-loop modes, dynamically switching between themodes of operation depending on the output signal amplitude. An exampleof such an amplifier design arranged to switch between open- andclosed-loop modes based on the signal magnitude can be seen in U.S. Pat.No. 9,628,040.

SUMMARY

Embodiments of the present disclosure relate to methods, apparatus andsystems for amplifier circuitry, for instance class-D amplifiercircuitry, that are selectively operable in open- and closed-loop modes.

According to an aspect of the disclosure there is provided amplifiercircuitry comprising:

-   -   a forward signal path for receiving an input signal and        outputting an output signal;    -   a feedback path operable to provide a feedback signal derived        from the output signal;    -   a feedforward path operable to provide a feedforward signal        derived from the input signal; and    -   a combiner operable to determine an error signal based on a        difference between the feedback signal and the feedforward        signal;    -   wherein the amplifier circuit is selectively operable in a        closed-loop mode of operation in which the input signal in the        forward signal path is combined with a signal derived from said        error signal and an open-loop mode of operation in which the        input signal in the forward signal path is not combined with a        signal derived from said error signal; and    -   wherein the feedforward path comprises a compensation module        configured to apply a controlled transfer function to the        feedforward signal in the closed-loop mode of operation such        that an overall transfer function for the amplifier circuit is        substantially the same in the closed-loop mode of operation and        the open-loop mode of operation.

The compensation module may comprise a filter having a controlledtransfer function.

In some implementations the compensation module may be an adaptivemodule, for example an adaptive filter. The compensation module may beconfigured to monitor the error signal and to adapt its transferfunction in response to the error signal, so that the error signalexhibits a desired characteristic over a signal band of interest.

In some implementations the compensation module may be configured tohave a transfer function which is matched to a transfer function for thefeedback path.

The amplifier circuitry may include a controller for selectivelycontrolling the amplifier circuit in the open-loop mode of operation orthe closed-loop mode of operation. The controller may be configured toselectively transition between the open-loop mode of operation and theclosed-loop mode of operation based on an indication of amplitude of theinput signal.

In some examples, the controller may be configured to selectivelycontrol a loop gain factor, wherein the loop gain factor is equal tozero in the open-loop mode of operation and equal to one in theclosed-loop mode of operation. The amplifier circuit may also beoperable in at least one additional mode of operation where the loopgain factor is equal to a non-zero value less than one.

In some examples the controller may be configured to control the loopgain factor based on an indication of power supply noise. In someexamples the controller may be configured to control the loop gainfactor based on the error signal.

In some examples the controller may be configured to control a biasapplied to a component in the feedback path to have a first level in theclosed loop mode of operation and a second, lower, level in saidadditional mode of operation.

The amplifier circuitry may be a class-D amplifier circuit. Theamplifier circuitry may have a class-D modulator and a class-D outputstage in the forward signal path.

In some implementations there may be a feedback filter in the feedbackpath. The feedback filter may, in some examples, be a low-pass filter.

The amplifier circuitry may include a loop-filter in an error path forthe error signal.

The amplifier circuitry may be implemented as an integrated circuit.

An aspect also relates to an electronic device comprising amplifiercircuitry as described in any of the variants herein.

In another aspect there is provided a class-D amplifier circuit forreceiving an input signal and outputting an output signal, the class-Damplifier circuit comprising a digital modulator and an output stage,wherein the class D amplifier is selectively operable in:

-   -   an open loop mode of operation wherein the input signal as        received is provided as an input to the modulator; and    -   a closed-loop mode of operation in which the input signal is        modified based on an error signal before being provided as an        input to the modulator, wherein the error signal is determined        based on a difference between a feedback signal derived from the        output signal and a feedforward signal derived from the input        signal; and    -   the class-D amplifier circuit further comprises a compensation        module, the compensation module having a transfer function        controlled such that an overall transfer function for the        class-D amplifier circuit in the closed-loop mode of operation        is substantially the same as in the open-loop mode of operation.

In a further aspect there is provided a class-D amplifier circuitcomprising:

-   -   a forward signal path for receiving an input signal and        outputting an output signal;    -   a feedback path for providing a feedback signal derived from the        output signal;    -   a feedforward path for providing a feedforward signal derived        from the input signal;    -   an error path for generating an error signal based on the        difference between the feedback signal and the feedforward        signal and modifying the input signal in the forward signal path        based on the error signal; and    -   a compensation module in the feedforward signal path having a        controlled transfer function so as to amply a compensation to        the error signal to compensate for a transfer function of the        feedback path.

In a further aspect, there is provided amplifier circuitry comprising:

-   -   a forward signal path for receiving an input signal and        outputting an output signal;    -   a feedback path operable to provide a feedback signal derived        from the output signal;    -   a feedforward path operable to provide a feedforward signal        derived from the input signal; and    -   a combiner operable to determine an error signal based on a        difference between the feedback signal and the feedforward        signal;    -   wherein the amplifier circuit is selectively operable to vary a        loop gain factor so as to vary an extent to which the error        signal contributes to the forward signal path;    -   wherein the feedforward path comprises a compensation module        configured to apply a controlled transfer function to the        feedforward signal such that an overall transfer function for        the amplifier circuit is substantially for different values of        the loop gain factor.

The amplifier circuit may be operable in full closed-loop mode with theloop gain factor equal to one. The amplifier circuit may be operable ina partial closed-loop mode with the loop gain factor between one andzero. The amplifier circuit may be operable in an open-loop mode withthe loop gain factor equal to zero.

It should be noted that, unless expressly indicated to the contraryherein or otherwise clearly incompatible, then any feature describedherein may be implemented in combination with any one or more otherdescribed features.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of examples of the present disclosure, and toshow more clearly how the examples may be carried into effect, referencewill now be made, by way of example only, to the following drawings inwhich:

FIG. 1 illustrates an example of a class-D amplifier circuit;

FIG. 2 illustrates an example system transfer function for the circuitof FIG. 1;

FIG. 3 illustrates an example of amplifier circuit according to anembodiment;

FIG. 4 illustrates an example system transfer function for the circuitof FIG. 1;

FIG. 5 illustrates a further example of amplifier circuit according toan embodiment;

FIG. 6 illustrates feedback with a full bridge output stage; and

FIG. 7 illustrates a further example of an amplifier circuit.

DETAILED DESCRIPTION

The description below sets forth example embodiments according to thisdisclosure. Further example embodiments and implementations will beapparent to those having ordinary skill in the art. Further, thosehaving ordinary skill in the art will recognize that various equivalenttechniques may be applied in lieu of, or in conjunction with, theembodiments discussed below, and all such equivalents should be deemedas being encompassed by the present disclosure.

Embodiments relate to amplifier circuitry and to methods of operationthereof that is selectively operable in a closed-loop mode of operation.

FIG. 1 illustrates one example of a general class-D amplifier circuit100. The amplifier circuit 100 receives an input signal S_(IN), whichwill typically be an input digital signal, and outputs an output signalS_(OUT) which may be a driving voltage. A forward signal path thusextends from an input node to an output node. This forward signal pathincludes a digital modulator 101, for instance a PWM modulator, upstreamof an output stage 102. The digital modulator 101 receives a signalderived from the input signal and generates a suitable modulator outputto control switches of the output stage 102. The output stage 102 may beimplemented with a half-bridge configuration so as to switch between twodifferent output states, e.g. a ‘High’ state (e.g. a +1 state) with theoutput connected to a high voltage V_(H) or a “Low” state (e.g. a −1state) with the output connected to a low voltage V_(L). In someinstances, however, the output stage could be implemented as afull-bridge arrangement, with two outputs that can be selectivelyswitched to provide a positive voltage differential (e.g. a +1 statewith a first output connected to the high voltage V_(H) and the secondoutput connected to the low voltage V_(L)) or a negative voltagedifferential (e.g. a −1 state with the first output connected to the lowvoltage V_(L) and the second output connected to the high voltageV_(H)). Additionally, some implementations may support an additionalstate with no voltage differential (e.g. a zero state with both outputsconnected to the same voltage V_(H) or V_(L)). In such a case themodulator 101 may be a suitable modulator for driving a full-bridgeoutput. Note as used herein the terms “high” and “low” used in thecontext of a voltage for the output stage as used in a relatively senseto distinguish between the two voltages and these terms do not imply anyparticular absolute voltage level. In some instance in someimplementations the high voltage could be a non-zero voltage of somemagnitude and the low voltage could be ground, in other implementationsthe high and low voltages could be of equal magnitude but oppositepolarity.

The amplifier circuit 100 is operable in a closed-loop mode of operationand thus includes a feedback path for providing a feedback signal S_(FB)derived from the output signal S_(OUT). The feedback path includes ananalogue-to-digital converter (ADC) 103. In some instances the feedbackpath may include a feedback path filter 104. The feedback path filtermay be implemented in the analogue part of the feedback path (asillustrated) or in the digital part of the feedback path.

In the example of FIG. 1 the feedback signal is subtracted from aversion of the input signal S_(IN) (or vice-versa) to provide an errorsignal ε. In the example of FIG. 1 the feedback signal S_(FB) iscombined with a feedforward signal S_(FF) of the input signal bycombiner 105 to provide the error signal ε. Thus a feedforward pathextends from a node of the main forward signal path to the combiner 105.

The error signal ε is filtered by a loop filter 106 and, in closed-loopoperation, the filtered error signal is combined with the input signalS_(IN) upstream of the modulator 101 by combiner 107. An error signalpath thus extends between the combiner 105 and the combiner 107 whichincludes the loop filter 106.

The action of the feedback loop can help to suppress any distortionsarising from analogue effects in the output stage 102 and/or anyvariation in the voltages V_(H) and/or V_(L). However, as mentionedabove, the performance of the ADC 103, for example the noisecharacteristics, also has an impact on the performance of the amplifier.In addition, the ADC 103 and other components of the feedback path addto the power consumption of the amplifier circuit 100 and generallythere is a desire for low power consumption.

The amplifier circuit 100 may thus be selectively operable in aclosed-loop mode of operation and also an open-loop mode of operation.The amplifier circuit may, for example, be operated in a closed-loopmode of operation at relatively high signal amplitudes. At relativelyhigh signal amplitudes, the effect of any distortion could be relativelysignificant, but the noise floor of the amplifier is less of a concerndue to the high signal amplitudes. It can therefore be beneficial tooperate in the closed-loop mode. At relatively lower signal amplitudesthe amplifier circuit may be operable in an open-loop mode of operation.At relatively low signal amplitudes the impact of any output stagedistortion may be relatively low, however for these lower signalamplitudes the noise floor of the amplifier is of greater concern.Therefore at low signal amplitudes it may be beneficial to operate in anopen-loop mode so as to avoid any noise contribution from the ADC 103.Such operation may allow the design tolerances for the ADC 103 to bemore relaxed than would be the case if the amplifier always operated ina closed-loop mode, without a degradation in performance at lower signalamplitudes, which can allow the ADC to be smaller in area and/or consumeless power. Such operation may also provide better performance at highersignal amplitudes than would be the case if the amplifier were alwaysoperated in an open-loop mode.

The amplifier circuit 100 therefore includes a controller 108 forcontrolling the amplifier circuit. In the example of FIG. 1 thecontroller monitors the input signal S_(IN) and, based on the inputsignal, selectively controls the amplifier circuit 100 in an open-loopmode of operation or a closed-loop mode of operation. For example, asillustrated in FIG. 1, the controller 108 may monitor the input signalS_(IN) to determine an indication of amplitude and may control aneffective gain of the loop filter 106, i.e. a gain of the transferfunction H of the loop filter 106. In the closed-loop mode of operationthe gain of the loop filter may be set to a predetermined non-zerovalue, e.g. H=H_(C). The amplifier circuit 100 thus operates asdescribed above and combined the filtered error signal with the inputsignal S_(IN). In the open-loop mode of operation the gain of the loopfilter may be set to zero, e.g. H=0. In this case there is effectivelyno contribution from the filtered error signal to the input signal, andthe input to the modulator 101 is simply the input signal. Thecontroller may thus monitor the input signal S_(IN) against one or morethresholds and selectively control the gain from the feedback, e.g. toswitch between the closed-loop and open-loop modes of operation based onan indication of signal amplitude.

It will be understood the controller could control the amplifier circuitto transition between open- and closed-loop modes of operation in otherways. For instance if an indication of signal amplitude is available,e.g. from upstream module, the controller 108 could receive such anindication. The effective gain of the loop-filter 106 could becontrolled by varying some components of the loop filter, or bycontrolling some digital gain element (not separately illustrated) inthe error path. Alternatively the combiner 107 for combining thefiltered error signal with the input signal S_(IN) could be a selectivecombiner which could be controlled to vary the extent to which thefiltered error signal is combined with the input signal.

In operation the amplifier circuit 100 may thus vary the effective loopgain to transition between the open- and closed-loop modes of operation.

The operation of the modulator 101 and output stage 102 collectivelyprovides a transfer function G in the main forward signal path. The ADC103 and filter 104 if present collectively provide a transfer function Min the feedback path. The loop filter applies a transfer function H inthe error path. If the input signal is represented by X and the outputsignal is represented by Y, the system transfer function (STF) will, inthe closed-loop mode of operation, be given by:

$\begin{matrix}{\frac{Y}{X} = \frac{G - {GH}}{1 - {GHM}}} & {{Eqn}.\mspace{14mu}(1)}\end{matrix}$

As noted above, the transfer function of the digital gain modulator 101may be substantially flat over the signal band of interest and typicallythe modulator applies no gain. The output stage 102 replicates themodulator output and thus, for many class-D amplifiers, the transferfunction G in the main forward signal path may be substantially equal tounity. In which case the STF of the loop may become:

$\begin{matrix}{\frac{Y}{X} = \frac{1 - H}{1 - {HM}}} & {{Eqn}.\mspace{14mu}(2)}\end{matrix}$

Typically the value of H may be relatively high, to provide suitableclosed-loop operation. In which case the STF, for closed loop operation,can be approximated to:

$\begin{matrix}{\frac{Y}{X} \approx \frac{1}{M}} & {{Eqn}.\mspace{14mu}(3)}\end{matrix}$

Thus the STF for the amplifier circuit 100 can be approximated to 1/M inthe closed-loop mode of operation.

The transfer function M of the feedback path may, in at least someamplifier designs, not be simply unity over the signal band of interestin order to provide desired operation in the closed-loop mode ofoperation. For example, in at least some designs filter 104 may bepresent in the feedback path and configured as a low-pass filter, toprovide suppression of the PWM carrier and any other high frequencysignals and transients at the output, to avoid them being mixed down toaudio frequencies by imperfections of the ADC.

FIG. 2 illustrates an example of the STF for an amplifier circuit 100such as illustrated in the FIG. 1 operating in closed-loop mode, inwhich the transfer function M of the feedback path is that of asecond-order low-pass filter. It can be seen that the STF is not flatover the signal band of interest, which in this example may be a bandfor audio signals.

In the open-loop mode of operation however, there is no contributionfrom the error path, and hence also no contribution from the feedbackpath, and thus the STF depends on the transfer function G of the mainforward signal path. As noted above, the transfer function G for themain forward signal path may be effectively unity, for the signal bandof interest.

In this case, the STF for the amplifier circuit 100 will change as aresult of a variable gain applied to the feedback, i.e. a loop gain, forinstance to transition between the open-loop mode of operation and theclosed-loop mode of operation. Such a change in STF may result inartefacts in the output signal, e.g. for an audio signal the transitionbetween modes may result in audible artefacts such as pops or clickswhich are undesirable.

Embodiments of the disclosure thus relates to amplifier circuits and tomethods of operation thereof that at least mitigate the problems ofvarying the loop gain, e.g. for transitioning between open- andclosed-loop modes of operation.

FIG. 3 illustrates an amplifier circuit 300 according to an embodiment,in which similar components to those described above with reference toFIG. 1 are identified by the same numerals.

The amplifier circuit 300 again has a forward signal path between aninput for receiving an input signal S_(IN) and an output for outputtingan output signal S_(OUT), where the main forward signal path includes amodulator 101 and an output stage 102 and has a transfer function G. Asdiscussed in relation to FIG. 1 a feedback path, including an ADC 103and possibly other components such as a feedback filter 104,collectively with a transfer function M, provides a feedback signal tocombiner 105. The combiner 105 is operable to generate an error signal εbased on the difference between the feedback signal S_(FB) and afeedforward signal S_(FF) derived from the input signal S_(IN). An errorpath for the error signal ε includes loop filter 106. Combiner 107 isoperable to combine the filtered error signal with the input signalS_(IN) upstream of the modulator in a closed-loop mode of operation.

The amplifier circuit 300 also includes a compensation module 201, forinstance a filter, which compensates for the transfer function M of thefeedback path. In the amplifier circuit 300 illustrated in FIG. 3 thecompensation module is located in the feedforward path so as to apply atransfer function B to the tapped version of the input signal S_(IN) toprovide the feedforward signal S_(FF).

The system transfer function (STF) for the amplifier circuit 300, inclosed-loop operation, will be given by:

$\begin{matrix}{\frac{Y}{X} = \frac{G - {GHB}}{1 - {GHM}}} & {{Eqn}.\mspace{14mu}(4)}\end{matrix}$

Again the transfer function G of the main forward signal path may besubstantially equal to unity across the signal band of interest. In thiscase, the closed-loop STF can be seen as:

$\begin{matrix}{\frac{Y}{X} = \frac{1 - {HB}}{1 - {HM}}} & {{Eqn}.\mspace{14mu}(5)}\end{matrix}$

In this case, if the transfer function B of the compensation module,e.g. filter, 201 is matched to that of the feedback path, the STF forclosed loop operation will also be equal to unity across the signalband. Thus the STF for the amplifier will be substantially the same inboth the open- and closed-loop modes of operation and any variation inloop gain, e.g. the gain applied to the error signal, will not alter theSTF of the amplifier circuit 300.

FIG. 4 illustrates an example of the STF for an amplifier circuit 300such as illustrated in the FIG. 3 operating in closed-loop mode, inwhich the transfer function M of the feedback path is that of asecond-order low-pass filter. FIG. 4 shows, in the dotted line, the STFif the transfer function B of the compensation module 201 were unity,i.e. with no compensation applied and, in the solid line, where thetransfer function B of the compensation module 201 is matched to that Mof the feedback path, i.e. with compensation applied. It can be seenthat with the appropriate compensation applied, the STF is flat over thesignal band of interest, which in this example may be a band for audiosignals. The STF for the amplifier circuit 300, with appropriatecompensation applied, is thus the same in both the open- and closed-loopmodes of operation. Therefore transitioning between the modes ofoperation results in no significant change in the STF for the amplifiercircuit and hence the possibility of any artefacts, such as audible popsor clicks in an audio signal, is thus significantly reduced or eveneliminated.

In some implementations the compensation module 210 may have apredefined transfer function B which may be defined with respect totransfer function M of the relevant components of feedback path, e.g.the filter 104 and ADC 103 as appropriate.

The transfer function M of the relevant components of the feedback pathmay be determined in any number of ways. The transfer function M couldbe determined by calculation or simulation based on the known design ofthe amplifier circuit, and/or the transfer function could be measured ina calibration step during device fabrication. The compensation module201 may, as discussed, comprise a digital filter. The digital filter maybe designed to have a desired transfer function in any of a number ofknow ways, for instance the compensation module may implement a functionbased on a polynomial and the coefficients of the polynomial may bedefined to provide the desired transfer function.

In some instances the compensation module 201 may comprise a filter witha predefined response. In some embodiments, however, the compensationmodule may comprise an adaptive filter which is configured to track theerror signal with a slow time constant. FIG. 5 illustrates an examplewhere the compensation module is adaptive, e.g. comprises an adaptivefilter, and is configured to track the error signal.

In this case the adaptive filter of the compensation module 201 isconfigured to adapt the transfer function B applied so that the errorsignal ε, over time, has a desired characteristic, e.g. is flat over thesignal band of interest. This adaption will effectively correct for anyeffects of the feedback path and thus will mean that the overall STF forthe amplifier circuit 300 in the closed loop mode of operation iseffectively flat and matched to the STF in the open-loop mode ofoperation.

FIG. 5 illustrates that the adaptive compensation module may track theerror signal ε before filtering, but any error node, i.e. a node thatvaries with the error signal could be monitored, which could be a nodeof the error path after the loop filter or possibly a node of theforward signal path.

In operation the controller 108 may thus operate to vary the mode ofoperation between the closed-loop mode of operation and the open-loopmode of operation according to operating conditions, for instance thesignal amplitude of the input signal S_(IN) as discussed above. Becausethe STF of the amplifier circuit does not substantially vary betweenthese operating modes, the controller 108 may be able to transitionbetween modes more quickly, or with less fading (e.g. intermediate gainsbeing applied by loop filter 106 or a digital gain element in the errorpath) than otherwise would be the case. It should be understood howeverthat the principles described herein apply to any switch in modes forany reason, and the controller 108 may additionally or alternatively beconfigured to selectively control the amplifier circuit to operate inthe closed-loop mode or the open-loop mode for any reason.

The examples above have described that the compensation module 201 ispart of the feedforward path. In theory if only a gain correction wereneeded, the compensation could be applied to the feedback path in thedigital domain after the ADC, but in practice there may be a need tomatch delay etc. and thus the compensation is advantageously applied inthe feedforward path.

The examples above have been described with reference to a single outputsignal for ease. As noted above however in some implementations theoutput stage 102 could be implemented as a full-bridge, e.g. to drive aload as a bridge-tied load. The same principles apply. In such a casethe feedback signal is derived from the voltage difference across theoutputs. For example, FIG. 6 illustrates just selected components of anamplifier circuit with an output stage 102 that comprise a full bridgefor outputting an output signal to a bridge-tied load (not illustrated).In this case the output signal comprises components S_(OUTa) andS_(OUTb) which are applied to either side of the load. The feedbacksignal S_(FB) may be determined by monitoring each output signalcomponent, for instance by separately converting each output signalcomponents to digital via a respective ADC 103 a and 103 b, possiblyafter filtering by a respective filter 104 a or 104 b. The differencebetween the two digital signals output from the ADCs 103 a and 103 b mayprovide the feedback signal. In this case the relevant transfer functionM of the feedback path is the transfer function between the outputsignal S_(OUTa)−S_(OUTb) and the feedback signal S_(FB).

The description above has focussed on the amplifier circuit 300transitioning between a closed-loop mode of operation and an open-loopmode of operation. In effect, a loop gain factor, which controls theextent to which any feedback is applied to the forward signal path maybe varied between 1 in the closed-loop mode of operation and 0 inopen-loop mode of operation. In the examples above, where gain may bevaried in the error path for the error signal ε, the error path may becontrolled to have a transfer function equivalent to αH_(Max), where αis the loop gain factor that is 0 in the open-loop mode and 1 in theclosed-loop mode and H_(Max) is a maximum of the transfer function ofthe error path.

In some implementations the amplifier circuit 300 may be operable in amode in which the loop gain factor α is controlled, in use, to anintermediate value between 1 and 0, and not just as part of a gradualtransition between open-loop and closed-loop modes of operation. Asnoted above, applying the appropriate transfer function B to thefeedforward signal S_(FF) means that the STF for the amplifier circuitdoes not substantially with the loop gain, and thus the loop gain factormay be varied to any intermediate value between 0 and 1 without anysubstantial impact on the STF of the amplifier circuit. Varying the loopgain factor does, however, vary the noise transfer characteristic andmay allow benefits in noise performance and/or power consumption.

The amplifier circuit 300 may, for instance, be operable in a first modewith the loop gain factor α equal to 1, which may be regarded as a‘full’ closed-loop mode of operation. In another mode of operation, theloop gain factor α may be controlled to some non-zero value less than 1.This mode of operation, may be regarded as a ‘partial’ closed-loop modewhere there is still some feedback to the forward signal path, but thecontribution of the feedback is reduced compared to the full closed-loopmode. The amplifier circuit may be operable in a partial closed-loopmode in addition to, or as an alternative to, operation in an open-loopmode with the loop gain factor α set to zero.

As discussed above, operating in the full closed-loop mode of operationmay be advantageous to suppress analogue distortion or THD in the outputstage and/or noise such as power supply noise but, for some operatingconditions these issues may be less of a concern. As discussed,distortion is more of a concern at higher signal levels and, as forclass-D amplifiers PSSR is multiplicative (rather than additive as isthe case for linear amplifiers), power supply noise is also less of aconcern at lower signal levels. For some operating conditions, e.g. atlower signal levels, the benefits of full closed-loop operation may beoffset by the noise and/or power consumption of the components of thefeedback path, e.g. the ADC 103. Switching to an open-loop mode ofoperation for certain operating conditions, such as when amplifyingrelatively low-level signals, can provide benefits in power consumptionand/or noise performance. Operating in a partial closed-loop mode ofoperation may allow further benefits in noise performance and/or powerconsumption compared to just operating in a full closed-loop mode or anopen-loop mode.

The controller 108 of the amplifier circuit 300 may thus be operable tocontrol a loop gain factor α applied to provide a partial closed-loopmode of operation.

FIG. 7 illustrates an example of an amplifier circuit 300, similar tothat illustrated in FIG. 3, which shows that the controller 108 may beconfigured to control the loop gain factor α, in this example a loopgain applied to the path for the error signal ε. As discussed above, thegain may be controlled by varying the gain of the loop filter 106 itselfand/or a gain element in the error path (not separately illustrated),although other implementations are possible.

In some implementations the loop gain factor may be variable between arange of different possible intermediate values, i.e. the amplifiercircuit may be operable with the loop gain factor α equal to a number ofdifferent values between 0 and 1. The controller 108 may be operable toselectively vary, in use, the loop gain factor based on the input signaland may selectively vary the loop gain factor between different valuesin a continuous or stepwise manner.

Reducing the loop gain will reduce the extent to which any noise fromthe ADC 103 and/or other components of the feedback path contribute tothe forward signal path, but maintaining the loop gain above zero, i.e.providing some feedback, allows for suppression of THD and power supplynoise. Operating in a partial closed-loop mode can thus allow a tradeoff between power supply noise/THD and noise arising from the componentsof the feedback path. Thus operating in a partial closed-loop mode for acertain range of signal levels may provide better noise performance thanoperating in a full closed-loop mode or an open-loop mode.

In some examples the controller 108 may thus be configured to vary theloop gain factor α so as to provide a trade off between distortion/PSSRand feedback path noise so as to improve noise performance.

Additionally or alternatively, the amplifier circuit 300 may beconfigured so as to reduce a bias applied to at least one component ofthe feedback path, e.g. the ADC 103, with reducing loop gain factor soas to improve power efficiency. FIG. 7 illustrates that the controller108 may be configured to control a bias applied to the ADC 103.

As one skilled in the art will understand the noise performance of anADC 103 may depend on a bias current/voltage applied to one or morecomponents of the ADC 103. Generally, achieving a desired noiseperformance may require a certain degree of biasing and a lower amountof biasing may result in higher noise. In some implementations,therefore, if the amplifier circuit is operating in a full closed-loopmode, with the loop gain factor α equal to 1, a bias at a first levelmay be applied so as to achieve a desired noise performance for the ADC103. If, however, the amplifier circuit operates in a partialclosed-loop mode of operation, with a loop gain factor less than 1, thebias applied to ADC 103 may be lower than the first level, so as toreduce power consumption. Whilst applying a lower bias means that thenoise in the feedback path may be higher, the reduced loop gain factormay mean that the noise contribution to the forward signal path may notsignificantly increase.

The controller 108 may be configured to control the loop gain factorbased on an indication of signal level, e.g. a signal envelope value, ina similar manner as discussed above. The controller 108 may thereforereceive a version of the input signal S_(IN).

In some implementations however the controller may additionally oralternatively control the loop gain factor based on some indication ofthe extent to which feedback is required. For example, the loop gainfactor could be controlled, at least partly, based on an indicationN_(PS) of the extent of any noise on the supply voltage V_(H) and/orV_(L)., e.g. the amount of supply ripple. An indication of the powersupply noise N_(PS) could be determined in a variety of ways, as will beunderstood by one skilled in the art. If there is not much significantsupply noise that requires suppressing the loop gain factor may be setrelatively low, however if there is a large amount of supply noise theloop gain factor may be high.

In some implementations, the loop gain factor could be based on theerror signal ε. The error signal ε indicates the amount of error in theoutput signal and hence the extent to which feedback is required tocompensate. FIG. 7 illustrates the controller 108 could thereforereceive the error signal ε.

Embodiments thus relate to amplifier circuits, in particular class-Damplifier circuits that may be operable, in use, to vary a loop gain ofa feedback loop of the amplifier circuit. In some embodiment the loopgain may be varied to transition between an open-loop mode of operationand a closed-loop mode of operation. In some embodiments the loop gainmay be variable between two or more non-zero values so as to vary thecontribution of the feedback in a closed-loop mode of operation. Theamplifier circuit may be part of a transducer driving circuit. Theamplifier circuit may be part of an audio circuit for amplifying audiosignals.

Embodiments may be implemented as an integrated circuit which in someexamples could be a codec or similar. Embodiments may be implemented ina host device, especially a portable and/or battery powered host devicesuch as a mobile computing device for example a laptop, notebook ortablet computer, a games console, a remote control device, a homeautomation controller or a domestic appliance including a domestictemperature or lighting control system, a toy, a machine such as arobot, an audio player, a video player, or a mobile telephone forexample a smartphone. The device could be a wearable device such as asmartwatch. It will be understood that embodiments may be implemented aspart of a system provided in a home appliance or in a vehicle orinteractive display. The amplifier circuit may be an audio amplifierused to drive an audio transducer such as a loudspeaker or surface audiosystem, but it will be understood that the amplifier may be used todrive other transducers, e.g. a vibrational transducer such as a linearresonant actuator for the generation of haptic effects. There is furtherprovided a host device incorporating the above-described system.

The skilled person will recognise that some aspects of theabove-described apparatus and methods, for example the discovery andconfiguration methods may be embodied as processor control code, forexample on a non-volatile carrier medium such as a disk, CD- or DVD-ROM,programmed memory such as read only memory (Firmware), or on a datacarrier such as an optical or electrical signal carrier. For manyapplications, embodiments will be implemented on a DSP (Digital SignalProcessor), ASIC (Application Specific Integrated Circuit) or FPGA(Field Programmable Gate Array). Thus the code may comprise conventionalprogram code or microcode or, for example code for setting up orcontrolling an ASIC or FPGA. The code may also comprise code fordynamically configuring re-configurable apparatus such asre-programmable logic gate arrays. Similarly the code may comprise codefor a hardware description language such as Verilog™ or VHDL (Very highspeed integrated circuit Hardware Description Language). As the skilledperson will appreciate, the code may be distributed between a pluralityof coupled components in communication with one another. Whereappropriate, the embodiments may also be implemented using code runningon a field-(re)programmable analogue array or similar device in order toconfigure analogue hardware.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope.

As used herein, when two or more elements are referred to as “coupled”to one another, such term indicates that such two or more elements arein electronic communication or mechanical communication, as applicable,whether connected indirectly or directly, with or without interveningelements.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, or component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative. Accordingly, modifications, additions, oromissions may be made to the systems, apparatuses, and methods describedherein without departing from the scope of the disclosure. For example,the components of the systems and apparatuses may be integrated orseparated. Moreover, the operations of the systems and apparatusesdisclosed herein may be performed by more, fewer, or other componentsand the methods described may include more, fewer, or other steps.Additionally, steps may be performed in any suitable order. As used inthis document, “each” refers to each member of a set or each member of asubset of a set.

Although exemplary embodiments are illustrated in the figures anddescribed below, the principles of the present disclosure may beimplemented using any number of techniques, whether currently known ornot. The present disclosure should in no way be limited to the exemplaryimplementations and techniques illustrated in the drawings and describedabove.

Unless otherwise specifically noted, articles depicted in the drawingsare not necessarily drawn to scale.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the disclosureand the concepts contributed by the inventor to furthering the art, andare construed as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present disclosurehave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the disclosure.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the foregoing figuresand description.

To aid the Patent Office and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke 35 U.S.C. § 112(f) unless the words “means for” or“step for” are explicitly used in the particular claim.

The invention claimed is:
 1. Amplifier circuitry comprising: a forwardsignal path for receiving an input signal and outputting an outputsignal; a feedback path operable to provide a feedback signal derivedfrom the output signal; a feedforward path operable to provide afeedforward signal derived from the input signal; and a combineroperable to determine an error signal based on a difference between thefeedback signal and the feedforward signal; wherein the amplifiercircuit is selectively operable in a closed-loop mode of operation inwhich the input signal in the forward signal path is combined with asignal derived from said error signal and an open-loop mode of operationin which the input signal in the forward signal path is not combinedwith a signal derived from said error signal; and wherein thefeedforward path comprises a compensation module configured to apply acontrolled transfer function to the feedforward signal in theclosed-loop mode of operation such that an overall transfer function forthe amplifier circuit is substantially the same in the closed-loop modeof operation and the open-loop mode of operation.
 2. Amplifier circuitryaccording to claim 1 wherein the compensation module comprises a filterhaving a controlled transfer function.
 3. Amplifier circuitry accordingto claim 2 wherein the filter is an adaptive filter.
 4. Amplifiercircuitry according to claim 1 wherein the compensation module is anadaptive module configured to monitor the error signal to adapt itstransfer function in response to the error signal so that the errorsignal exhibits a desired characteristic over a signal band of interest.5. Amplifier circuitry according to claim 1 wherein the compensationmodule is configured to have a transfer function which is matched to atransfer function for the feedback path.
 6. Amplifier circuitryaccording to claim 1 comprising a controller for selectively controllingthe amplifier circuit in the open-loop mode of operation or theclosed-loop mode of operation.
 7. Amplifier circuitry according to claim6 wherein the controller is configured to selectively transition betweenthe open-loop mode of operation and the closed-loop mode of operationbased on an indication of amplitude of the input signal.
 8. Amplifiercircuitry according to claim 6 wherein the controller is configured toselectively control a loop gain factor, wherein the loop gain factor isequal to zero in the open-loop mode of operation and equal to one in theclosed-loop mode of operation, and wherein the amplifier circuit is alsooperable in at least one additional mode of operation where the loopgain factor is equal to a non-zero value less than one.
 9. Amplifiercircuitry according to claim 8 wherein the controller is configured tocontrol the loop gain factor based on an indication of power supplynoise.
 10. Amplifier circuitry according to claim 8 wherein thecontroller is configured to control the loop gain factor based on theerror signal.
 11. Amplifier circuitry according to claim 8 wherein thecontroller is configured to control a bias applied to a component in thefeedback path to have a first level in the closed loop mode of operationand a second, lower, level in said additional mode of operation. 12.Amplifier circuitry according to claim 1 comprising a class-D modulatorand a class-D output stage in the forward signal path.
 13. Amplifiercircuitry according to claim 1 comprising a feedback filter in thefeedback path.
 14. Amplifier circuitry according to claim 13 whereinsaid feedback filter comprises a low-pass filter.
 15. Amplifiercircuitry according to claim 1 comprising a loop-filter in an error pathfor the error signal.
 16. Amplifier circuitry according to claim 1implemented as an integrated circuit.
 17. An electronic devicecomprising amplifier circuitry according to claim
 1. 18. A class-Damplifier circuit for receiving an input signal and outputting an outputsignal, the class-D amplifier circuit comprising a digital modulator andan output stage, wherein the class D amplifier is selectively operablein: an open loop mode of operation wherein the input signal as receivedis provided as an input to the modulator; and a closed-loop mode ofoperation in which the input signal is modified based on an error signalbefore being provided as an input to the modulator, wherein the errorsignal is determined based on a difference between a feedback signalderived from the output signal and a feedforward signal derived from theinput signal; and the class-D amplifier circuit further comprises acompensation module, the compensation module having a transfer functioncontrolled such that an overall transfer function for the class-Damplifier circuit in the closed-loop mode of operation is substantiallythe same as in the open-loop mode of operation.